How can I make unique rocks in Blender without having to model them by hand?
Add Rocks

The Add Rocks addon allows you to quickly add randomly generated mesh rocks, thus making it much quicker to add detail &amp; realism to your outdoor settings. Here you can read about this addon.

Here are some examples of the presets and some variations:  



To enable this addon:


Download it from here.  
Now open User Preferences:
Info header  &rarr;  File  &rarr;  User Preferences  

  
Click on Addons at the top of the Preferences window, and then click on the Install from File button at the bottom of the window: 


Next:  


Find the addon file and select it. (NOTE: The file can be a .zip file -- you don't have to unzip it first.)
Click the Install from File button
Then enable the addon by clicking on it's checkbox on the far right
If you want this addon to be enabled by default in the future, click on the Save User Settings button:



To add a rock to your scene:
SHIFTA -> Mesh -> Rock Generator


Adjust all the settings:  

 

One very neat feature is that you can make as many rocks as you want all at once, each one unique, by setting the Number of Rocks at the top -- here I'm making 500 all at once:  



Notice at the very bottom, there are some presets:  



NOTE: You MUST set all the settings the way you want them to be before you do ANYTHING else, otherwise they cannot be changed through the same settings panel. After you click away from the rocks, you can only modify their parameters via the modifiers that are on them:
Object Properties panel -> Modifiers tab


Apply All Modifiers BEFORE Rendering

One thing to be careful of is that each rock created has many modifiers on it and these DRAMATICALLY slow down rendering speed! So, to get around this, you really need to Apply all modifiers BEFORE attempting to render your scene. To do this you can do either of the following:  


Manually Apply Modifiers:  This is an easy method, but it can take a lot of time if you have more than 1 rock:
Select rock -> Object Properties panel -> Modifiers tab -> click the Apply button on the modifier  


Convert All Rocks to Mesh: This is the best and quickest method if you don't want to sit around clicking "Apply" for a long time:
Select ALL the rocks at once -> ALTC -> Convert to Mesh  

This will very easily Apply ALL the modifiers without you having to do anything else and you can now render quickly and easily: